- Switch devel/sdl20 to use the autotools-based configuration, since the
cmake-based one is not maintained that well
- Fix library naming to ensure backwards compatibility on minor version
changes
- Bump the PORTREVISION on ports depending on devel/sdl20, since its
shared library name changed
PR: ports/183975
Submitted by: Jan Kokemuller <jan.kokemueller@gmail.com>

- Update devel/sdl20 to version 2.0.1
- Bump PORTREVISIONs for SDL2-related ports to chase the changed SDL2
shlib version
- Enable staging support for the SDL2 ports
- Switch to new LIB_DEPENDS format for the SDL2 ports
- Add LICENSE to the SDL2 ports
- Use gmake USES for the SDL2 ports

Welcome SDL2 to the FreeBSD ports system!
A set of new SDL2 related ports has been added, which are essentially
the same as their corresponding port for SDL1.2:
audio/sdl2_mixer
graphics/sdl2_image
graphics/sdl2_ttf
net/sdl2_net
A set of new USE_SDL knobs is available to enable SDL2 and its related
ports to be added as dependency to ports. Use either of
USE_SDL= sdl2 image2 mixer2 net2 ttf2
to pull in the specific sdl2_* or sdl20 port.